1 Introduction
Sawn timber needs to be strength graded prior to its use for structural applications. In Europe, grading is based on the allocation of timber to groups called strength classes. These classes are defined by sets of statistical “characteristic values” of key properties, that allow safe design with timber by specifying minimum requirements for those values, allowing them to be used for structural calculations. For this, it is first necessary to derive relationships to predict the properties, so that the grading can correctly take place. Normally the three key wood properties investigated are bending stiffness (Em), bending strength (fm) and density (ρ). The characteristic values for stiffness (Em,0,mean) are based on the mean, whereas for strength (fm,k) and density (ρk) it is based on the lower 5th percentiles. Thus, it is the collective properties within a grade that matter, and not the properties of individual pieces of timber.
Even though grading established on the basis of bending tests is the most common method, in Europe strength grading can also be determined based on tension testing (Ridley-Ellis et al.
2016). Depending on the types of loads to which the timber will be subjected in service, one basis of grading will be more efficient than the other. While bending strength is the most common basis for grading in timber construction, tension properties are still important for elements like trusses and I-joists. Interest in mass timber construction in recent years has also raised the profile of tension-based strength classes of lamellas for glulam manufacturing. In fact, the most recent version of EN338 (CEN
2016a) includes strength classes for tension grading of softwoods (T classes) in addition to the more familiar bending classes (C classes), although tension classes had been in use for more than a decade previously. Irrespective of the testing basis being bending or tension, the strength classes are based on the characteristic values of stiffness, strength and density. While grading could conceivably be based on both tension and bending testing, the cost of this would be prohibitive. For economic and practical reasons, it is also common to use timber graded on the basis of bending in situations where tension is the dominating load. Moreover, it is often the case that bending elements need to be designed for tensile load, and vice versa. This is why the other strength property can be conservatively estimated by empirical relationships (EN 384) even though this can result in less efficient use of wood depending on the situation.
The grading process, whether done by machine or visually, uses non-destructive assessment of the wood that is indicative of the three grade determining properties. One kind of long-standing indicating property (IP) used to predict wood properties is knot indices, which is familiar for visual grading, but also, in a different form, part of grading by machines using principles such as X-rays, and optical mapping of the wood surface. In recent years, the use of non-destructive techniques based on the measurement of dynamic modulus of elasticity (
Edyn) has also become broadly applied (Gil-Moreno et al.
2019b; Krajnc et al.
2019b; Ridley-Ellis et al.
2018) since it is relatively easy to apply and usually gives strong relationship with mechanical properties, even in logs and standing trees (Gil-Moreno and Ridley-Ellis
2015; Llana et al.
2020; Wang
2013). Knot index and
Edyn are used in the current study as representative examples of IP, but there are also others in common use.
The empirical relationships in the standards can change when standards are revised, especially when there is new test data to inform safe, effective grading. This changes the design values for strength classes, and may need previously calculated designs to be changed in the case of reductions. The standard EN338:2009 provided Eq. (
1) to determine characteristic tension strength parallel to the grain (
ft,0,k) based on characteristic bending strength (
fm,k), but the 2016 revision (EN384:2016, EN338:2016) changed this to a new equation, Eq. (
2), which (together with the rounding used in EN338) had the effect of reducing tension strength values below C22 and increasing them for higher grades. This revision also included the listing of softwood tension strength classes, and a corresponding equation, Eq. (
3), for calculating the characteristic bending strength from the characteristic tension strength. Note that the equations are conservative estimates of one property based on test results of the other (derived using the lower 75%-prediction line around the regression), and so Eq. (
2) and Eq. (
3) can only be applied in the direction as written.
$$f_{t,0,k} = 0.6f_{m,k}$$
(1)
$$f_{t,0,k} = - 3.07 + 0.73f_{m,k}$$
(2)
$$f_{m,k} = {3}.{66} + {1}.{213}f_{t,0,k}$$
(3)
The basis for the new equations was extensive test data on spruce and pine from the Gradewood project (Ranta-Maunus et al.
2011), summarised for the standards committee CEN TC124 WG2 in document N832, which used
Edyn and the TKAR knot index (total knot area ratio) as representative IPs in the analysis. For balance, only Gradewood data from countries where both bending and tension tests were carried out were used. This comprised 429 bending tests and 423 tension tests for Scots pine (
Pinus sylvestris L.), originating from Poland and Sweden, and 2615 bending tests and 1392 tension tests for Norway spruce (
Picea abies L.) originating from Poland, Romania, Sweden, Slovenia, Slovakia and Ukraine.
Wood properties vary from species to species, and by country (Lavers
2002). In the N832 report, the
fm,k values for each country ranged between 20 and 26 N/mm
2 (average 21.9 N/mm
2) for spruce, and 17–23 N/mm
2 (average 19.8 N/mm
2) for pine. It is common for spruce and pine from Scandinavia and Central Europe to achieve strength classes above C22 (Fischer et al.
2016,
2015; Hanhijärvi and Ranta-Maunus
2008; Høibø et al.
2013; Stöd et al.
2016). In Ireland and the UK, the main species grown for construction is Sitka spruce (
Picea sitchensis (Bong) Carr.). The species is typically graded for structural timber to C16 (
Em,0,mean = 8 kN/mm
2,
fm,k = 16 N/mm
2,
ρk = 310 kg/m
3), but actually has superior strength and density than this requires. Grading is limited by the stiffness (Moore et al.
2013), a common feature in other species in the growing region (Gil-Moreno et al.
2016a). The lower performance compared to species growing in continental Europe is largely influenced by the shorter rotation lengths used. Typically these are 35–45 years (Moore
2011) but can be shorter (Ni Dhubháin et al.
2006). This limits the growth of the outerwood, which has higher mechanical properties (Kliger et al.
1998; Moore et al.
2012). Sitka spruce has similar properties to Norway spruce, and these are graded together as a species combination known as British spruce, recognised in the standards EN13556 and EN14081-1 (species code WPCS). Timber from Ireland and the UK is often graded using the same rules (Gil-Moreno et al.
2019b) due to the similar growing conditions that result in comparable timber characteristics. Scots pine is a native minor species, and although it can perform better than spruce, and achieve high yields of C20 (Moore et al.
2008; Fátharta et al.
2020), there is currently little information on the wood properties of the species grown in these countries. The current study covers mostly material from Ireland but conclusions are expected to also apply to UK-grown material.
To the authors’ knowledge, only two studies have addressed and published the properties of Irish timber in tension (Ó Fátharta et al.
2020; Raftery and Harte
2014), and none in the UK. There are no approved grading settings in tension for these countries, and therefore the use of timber in tension relies on the relationships given in the European standards. However, previous experience in bending on British grown conifer species showed that extrapolating models derived from other sources may not fit the characteristics of Irish and UK timber (Gil-Moreno et al.
2016b). In tension, Gil-Moreno et al. (
2019a) showed that extrapolating equations derived from the Gradewood project to Irish timber reduced the grading yields.
Likewise, previous studies have found different ratios for the relationship between tension and bending strength. During the drafting of Eurocode 5, Green and Kretschmann (
1989) obtained an average
ft,0,k/fm,k ratio of 0.59 for different softwood species in the US and Canada, with slight changes above 55 N/mm
2 tensile strength. For radiata pine timber, Tsehaye et al. (
1997) compiled data from New Zealand and Australia for visual and machine grades that showed a variation of ratios between 0.3 and 0.55. Based on the brittle fracture theory, and mostly spruce from Central Europe, Burger and Glos (
1997) determined a ratio
ft,0,k/fm,k of 0.69 that increased for higher strengths. In parallel, using this data together with values from the literature, Burger and Glos concluded that tensile strength in EN338:1995 was overrated in the lower strength classes and the contrary for higher strength classes. On Danish-grown Sitka spruce, for a range of
fm,k values between 21.7 and 33.6 N/mm
2, and
ft,0,k between 17.1 and 24.4 N/mm
2, Bräuner et al. (
2000) found ratios
ft,0,k/fm,k of 0.66, 0.77 and 0.81 for visual grades T1, T2 and T2 + (from lower to higher quality). More recently, using structural mechanical tests on Norway spruce from Central Europe, Steiger and Arnold (
2009) determined that the
ft,0,k/fm,k ratio depends on timber quality, and based on a linear regression where 27 <
fm,k < 47 N/mm
2 concluded a ratio smaller than 0.6 for
fm,k below 22 N/mm
2.
Taking into account the variation in the relationships between wood properties with the sampling sources (Denzler
2012; Stapel and Denzler
2010), and the ratios between tension and bending properties depending on the timber quality, the current study hypothesises that the equation in EN384:2016 underestimates excessively the performance in tension of the lower bending grades in EN338. In fact, N832 states that the old conversion Eq. (
1)
is generally agreed to be correct for lower grades. The aim of this study is to investigate the relationships between bending and tension characteristic strength of lower-grade timber. The study uses British spruce and Scots pine grown in Ireland and the UK tested in bending and tension. The analysis uses IPs thresholds to create bending and tension subsets of equivalent quality on which to obtain pairs of characteristic values to determine the relationship. Secondly, alternative equations to those given in EN384 are examined combining the current data with those from Scandinavia and Central Europe. This knowledge is particularly important in timber engineering for the optimisation and characterisation of timber used for trusses, I-joist and glulam manufacturing.
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